1. Field of the Invention
The present invention relates generally to a charge-coupled device (CCD) and a process for fabricating the same. More specifically, the invention relates to a CCD employing a floating gate amplifier, a distributed floating gate amplifier, or so forth, with a holding capability for controlling threshold voltage at peripheral elements and for preventing pin hole phenomenon.
2. Description of the Background Art
As is well known, CCD's have been principally used for solid state image pick-up devices, semiconductor delay lines and so forth. A CCD is generally composed of a charge transfer section for transferring signalling charge, and a section forming its peripheral components. In most cases, the peripheral components in a CCD comprise MOS transistors. CCD's have various output gate structures, such as a floating diffusion amplifier, a floating gate amplifier, a distributed floating gate amplifier, and so forth.
In such CCD's, gate insulation layers are typically formed by silicon oxide layers. Such silicon oxide layers tend to cause a pin hole phenomenon in the charge transfer section. In order to prevent this undesirable effect, there has been proposed a multi-layer structure having an insulating layer composed of a laminated silicon oxide layer, a silicon nitride layer, and a silicon oxide layer, referred to generally as an ONO structure. Such an ONO structure insulating layer is successful in prevention of the pin hole phenomenon. On the other hand, such an ONO structure insulating layer makes it difficult to obtain a desired threshold voltage at the peripheral components. Therefore, the ONO structure type insulating layer is not at all satisfactory for obtaining a desired performance of the CCD.
In order to solve the aforementioned drawbacks in the sole silicon oxide and ONO structure insulating layer, it is one possible approach to form the insulating layer by a combination of the sole silicon oxide insulating layer and the ONO structure insulating layer. In this possible combination, the ONO structure insulating layer is employed beneath the charge transfer section and the sole semiconductor insulating layer, such as a silicon oxide layer, is employed beneath the other electrodes, such as the gate electrodes of the peripheral components. With such construction, the aforementioned drawback may be resolved. However, on the other hand, such an insulating structure may cause difficulty in reading out a signal beneath the floating gate electrode and in smoothly transferring the signal through the transferring electrode. This is caused by discontinuity of the insulating layers.
Accordingly, this invention is directed to overcoming the drawbacks in the art relating to such CCDs.